The mechanism of cycloaddition reaction between singlet dichlorovinylidene (R1) and formaldehyde (R2) has been investigated with MP2 and B3LYP /6-31G* methods, including geometry optimization, vibrational analysis, and energy for the involved stationary points on the potential energy surface. Energies from both methods are also further corrected by CCSD(T)/6-31G* single-point calculations. Although the relative energies do differ especially for the loose conformations such as transition states and intermediates, generally the geometries predicted by MP2 and B3LYP are in good agreement. CCSD(T) relative energies for the stationary points predicted by MP2 and B3LYP agree quite well, and they are more comparative to those from B3LYP than those from MP2. The results also show that both three-centered and [2+2] cycloadditions can happen in concerted pathways. The former leads to a stable three-membered ring product (PI), while the two intermediates (INT1c and INT1d) from the latter are not so stable and will rearrange into either PI or a more stable four-membered ring product (P2). The orbital interactions are also discussed for the leading intermediates and products.